Although interventional
electrophysiology and the use of radiofrequency energy to cure various
arrhythmias primarily developed in the adult population, similar
applications in children have grown dramatically over the last decade.
The anatomic basis for various arrhythmias is critically important for
the pediatric ablationists to appreciate. Such understanding allows the
use of alternative technique to affect cure while avoiding
complications. Further, because of the relatively small heart and less
thick myocardium in children, without the appreciation of the
underlying cardiac anatomic relationships, collateral injury, for
example to the arterial system, may occur. In this review, the cardiac
anatomic consideration important in approaching various
supraventricular and ventricular arrhythmias in the normal heart is
discussed.

The last two decades have
witnessed tremendous change in the care of children with symptomatic
cardiac arrhythmia. The anatomic basis for various cardiac arrhythmias
(in children and adults) has been much better understood, particularly
for common supraventricular tachyarrhythmia.
While cardiac
electrophysiology still forms the basis for electrophysiological study
and radiofrequency ablation for supraventricular arrhythmia, pure
anatomic approaches are evolving with appropriate adjunctive
electrophysiological maneuvers performed prior to energy delivery.
Radiofrequency ablation for AV node reentry and paroxysmal atrial
fibrillation is performed almost entirely as an anatomy-based ablation
in many centers.1,2
Radiofrequency
ablation was initially reported as a feasible, successful, and safe
therapy that could be used in children as an alternative to drug
therapy in 19913. There are,
however, significant challenges when performing the sometimes complex
procedures in children, particularly the very young. An approach to
some of these difficult scenarios encountered in pediatric cardiac
electrophysiology is the subject of another manuscript in this series.
In this paper, I will describe the anatomic basis for common
arrhythmias with specific emphasis on variation in the pediatric
population relevant to safe and affective ablation in these patients.
We will confine ourselves to "normal" cardiac anatomy and the genesis
of arrhythmia. In another paper found in this supplement, we will also
cover an approach to the anatomy and ablation difficulty in children
who had surgically corrected congenital heart disease.

AV Node Reentrant Tachycardia
(AVNRT)

Over 90% of
supraventricular arrhythmia in children is a result of a reentrant
mechanism.4 The second most
common reentrant supraventricular tachycardia (SVT) in children is AV
node reentrant tachycardia. This arrhythmia is relatively less common
in the very young and preadolescent child but becomes increasingly more
prevalent in older children and teenagers. There are several reasons
why an accurate understanding of the anatomic basis of this arrhythmia
is important for pediatric electrophysiologists. These include the need
to ablate in the vicinity of the normal conduction system in these
patients with relatively small hearts (increasing the risk of heart
block) and the need to distinguish this arrhythmia from other common
arrhythmias in childhood including reentrant SVT using a septal bypass
tract and adjunctional tachycardia.

Anatomic Basis
of AVNRT

In the initial
few decades of cardiac electrophysiology, AVNRT was thought to
represent a reentrant arrhythmia arising entirely from the compact AV
node. This was because there was nearly simultaneous activation of the
ventricle and atrium observed from the earliest recorded demonstration
of this arrhythmia. If this were to have been the case (entire circuit
in AV node), radiofrequency ablation for this arrhythmia without heart
block would not have been possible. It later became recognized that AV
node reentry could be reset and entrained from the atrium particularly
the posteroseptal annular atrial tissue (Figure 1). Importantly, resetting of
the tachycardia could be demonstrated without
necessarily effecting the His bundle electrogram, yet either advancing
or delaying the subsequent atrial electrogram.2
Pioneering work such as this gave rise to the notion that ablation in
the atrium could potentially represent a definitive treatment for this
arrhythmia as has turned out to be the case.
The
circuit for this arrhythmia, however, is far from completely
understood. While it is accepted that atrial inputs to the AV node are
necessary components to the circuit (see below), certain less commonly
observed phenomena do suggest a distinct difference from AVNRT and any
other macroreentrant atrial tachycardia. AVNRT can be dissociated from
the ventricle either with block occurring in an infra-Hisian location
(more common) or above the bundle of His. Also clearly observed and
well documented are cases of AV node reentry with dissociation of the
tachycardia from all recorded atrial tissue (upper common pathway
block, see below).

Figure 1: Illustration of
the atrial input to the compact AV node. Posterior myocardium enters
the AV node involving atrial myocardium of the coronary sinus and
between the anterior rim of the coronary sinus and the tricuspid
annulus. These fibers constitute the slow pathway, and when entering to
the compact AV node, the propagated impulse travels simultaneously to
the atrial myocardium via the anterior positioned fast pathway and
through the lower AV node and His bundle to the ventricle. This
simultaneous disbursement of the impulse gives rise to the very short
R-P interval during AV node reentrant tachycardia.

What is the
Circuit for AVNRT?

The
sinus impulse originates epicardially at the junction of the crista
terminalis and the superior vena cava, where as the atrioventricular
conduction system is located anteriorly and septally on the annulus.
Specifically, the penetrating bundle of His is consistently found at
the junction of the noncoronary cusp and right coronary cusp of the
aortic valve where it meets the commissure of the anterior and septal
leaflets of the tricuspid valve (membranous intraventricular septum).
The compact AV node itself is located relatively more atrial and more
posteriorly (closer to the coronary sinus) in the mid-septal tricuspid
annulus when compared to the penetrating bundle of His. Although the
compact AV node is relatively atrial to the His bundle, it is always
found ventricular to the Eustachian ridge and the underlying tendon of
Todaro within the so-called triangle of Koch. This triangle is bounded
anteriorly by the tricuspid annulus, posteriorly by the tendon of
Todaro, and inferiorly by the coronary sinus.
The
atrial inputs into the AV node that carry the sinus node impulse (or
paced impulse) are not diffuse but along distinct anatomic pathways.
This is because of the anatomic obstacles in the atrium (particularly
right atrium) that the impulse must circumvent to reach the AV node.
The atrial myocardial input to the AV node that occurs superior to the
fossa ovalis and through the apex of the triangle of Koch is referred
to as the fast pathway. The posterior inputs to the AV node traverse to
this structure along the interatrial septum. These posterior inputs may
involve the myocardial sleeves along the coronary sinus and the
posteroseptal right atrium in the region of the coronary sinus ostium.
The posterior inputs are referred to as the slow pathway and may be
more than one in number (right and left-sided slow pathways). The
actual junction of these posterior inputs into the AV node is likely
through extensions of compact AV nodal and transitional AV nodal tissue
called the posterior horns. This anatomic distinction between the
atrial myocardial inputs to the AV node forms the essential substrate
for AV node reentrant tachycardia.
It
should be noted that the distinction between the fast and slow
pathways, while although partially reflective of the different
conduction and refractory properties, is fundamentally an anatomic
distinction (Figure 2). In
other words, regardless of conduction time and H-A interval, anterior
inputs to the AV node constitute the fast
pathway.

Figure 2: Autopsied
heart dissected and shown in the right anterior oblique view (Right
atrium and right ventricle). The fossa
ovalis and coronary sinus constitute anatomic obstacles to the wave
front from the high right atrium (sinus node, pacing) to reach the
compact AV node. Transmission anterior and superior to the fossa ovalis
anatomically constitutes the fast pathway input to the AV node, whereas
the propagated wave front posterior and ventricular to the fossa ovalis
in the region of the coronary sinus constitutes the anatomic slow
pathway (see text for details).

This
anatomic
distinction of the fast and slow pathway is not easily ascertained in
the antegrade direction and involves complex entrainment maneuvers
during AV node reentry to provide evidence. On the other hand,
retrograde activation of the atrium during AV node reentry or
ventricular pacing is straightforward in demonstrating proof of the
anatomic separation of the two limbs ofthe AVNRT
circuit. During typical AV node reentry or during ventricular
pacing with retrograde activation of the fast pathway, the earliest
recordable site of atrial activation is just behind the tendon of
Todaro, close to the apex of the triangle of Koch. Whereas with
atypical AV node reentry or retrograde activation via the slow pathway
during ventricular pacing, the earliest recorded atrial electrograms
are in the region of the coronary sinus ostium (posterior).
The fact that
atrial myocardium is essential to the
circuit of AV node reentry is more than semantics and forms the basis
for all atrial AVNRT ablation procedures. While ablating either
the anatomic fast or slow pathway will prevent future AVNRT, the exit
site of the fast pathway is fairly close to the compact AV node and
separated only from this structure by the tendon of Todaro. The
slow pathway, however, (see below) can be ablated 4 of 5 cm from the
compact AV node with little or no risk of AV block. This
distinction in the anatomy of the two pathways to the AV node is
critical for safe ablation in young children.

Fast Pathway

It is
important for ablationists to understand the
fluoroscopic anatomy of the fast pathway. In the RAO projection
as anticipated, the fast pathway site is more atrial (behind the
Eustachian ridge) than the compact AV node and, of course, the
penetrating bundle of His. The characteristic fluoroscopic
appearance, however, when mapping the fast pathway, is best noted in
the LAO projection (Figure 2).
As the catheter is moved
posteriorly (more atrially from the His location) with clockwise torque
being applied on the catheter, there will be a sudden leftward movement
of the catheter almost as if the fossa ovalis has been reached. This is
because the Eustachian ridge keeps any catheter that is
ventricular to the structure relatively rightward in the LAO
projection, thus, a His bundle catheter (ventricular to the Eustachian
ridge) will be more rightward than the fast pathway catheter (posterior
to the Eustachian ridge). In typical AV node reentry, a catheter
appropriately positioned in the fast pathway will record an earlier
atrial electrogram than the atrial electrograms on the His bundle
catheter itself.

Anatomy of the Slow Pathway

The slow pathway fibers are funnel-like and
triangular in section with the apex of the triangle inserting into the
compact AV node and the base fanning out into the posterior interatrial
septum both involving the fibers of the coronary sinus and the right
posterior septal atrium (Figure 3).
Thus, to ablate the slow
pathway, either a single point ablation, relatively close to the
compact AV node or a relatively long linear lesion at the level of the
floor of the coronary sinus ostium is effective.5

Figure 3:
Fluoroscopic images illustrating the anatomic
difference between the His bundle and the fast pathway. The left
panel is the right anterior oblique (RAO) projection and the right
panel is the left anterior oblique (LAO) projection. Note, in the
LAO view, the quadrapolar ablation catheter points leftward when
compared to the octapolar His bundle recording catheter placed on the
septum. This is because the fast pathway being mapped by this
catheter is behind the tendon of Todaro, allowing the catheter to point
in a leftward manner. Accurate mapping of the fast pathway is
important when ablating various supraventricular arrhythmias with early
atrial activation on the atrial septum in children (see text for
details).

Applications in Pediatric
Electrophysiology

1. Because of the relatively smaller hearts and thus
closer proximity of the compact AV node to the atrial input sites,
ablation in children should be performed as far away from the compact
AV node as possible.6,7
Understanding the anatomy of the AVNRT
circuit, this can be effectively accomplished with a complete linear
lesion at the level of the floor of the coronary sinus ostium. Because
of the left-sided inputs to the AV node as well, ablation often
is required slightly within the coronary sinus on the floor.8
Care should be taken when ablating using this approach not to ablate on
the roof of the coronary sinus near the ostium (compact AV node) and to
avoid inadvertent cannulation of the middle cardiac vein. A
catheter that has cannulated the middle cardiac vein is easily
recognized when counter-clockwise torque is applied in the LAO
projection. A catheter on the slow pathway region will
immediately move rightward and away from the septum, but such movement
is not possible and the tip will remain engaged when in the middle
cardiac vein.
2. Because of relatively rapid conduction times in
children, it can be difficult to distinguish whether retrograde
activation is via the slow pathway or fast pathway. Thus, there
is often simultaneous activation of the atrium as recorded on the His
bundle catheter and the proximal coronary sinus catheter. Specific
mapping, understanding the fluoroscopic anatomy as detailed
above of the fast pathway will clarify the situation.
3. In children who have a persistent left superior
vena cava, the coronary sinus including the ostium can be very
large. This can make ablation of the slow pathway
difficult. This difficulty is because of the lack of catheter
stability created by the large coronary sinus ostium and the fact that
multiple slow pathway inputs related to the diverse musculature of the
enlarged coronary sinus will require attention. Again, linear
ablation including careful ablation within the coronary sinus is most
likely to be effective.9
4. In children, because of relatively rapid
conduction times through the fast pathway, typically AVNRT may present
as a long R-P tachycardia. This is because in AVNRT, activation
of the atria and ventricle occur from a common turnaround point likely
within the compact AV node or next to it (slower common pathway). When
fast pathway activation is rapid, then atrial activation proceeds
ventricular activation, giving rise to a long R-P interval. The P
wave will be narrow and negative in leads 2, 3, and aVF unlike the wide
P waves that result from slow pathway activation (proximal coronary
sinus). Once catheters are placed, specific mapping of the fast pathway
will show earlier activation than the anatomic slow pathway region
(proximal coronary sinus).
5. Activation of the left atrium in sinus rhythm or
any right atrial rhythm occurs primarily through Bachmann's bundle and
through the coronary sinus musculature. In children, Bachmann's
bundle activation can be rapid. This occurrence can give rise to
unusual activation patterns in the coronary sinus in children with
typical AVNRT (Figure 4). If
one follows the circuit of typical
AVNRT, antegrade activation is via the posterior input of the AV node
(slow pathway), and from the common turnaround point in the AV node,
atrial activation breaks through just behind the tendon of Todaro at
the fast pathway region. To complete the circuit, activation must
now precede from the fast pathway into the slow pathway region either
crossing the Eustachian ridge (at times a line of conduction block) or
skirting around it between this ridge and the crista terminalis. In
children (and some adults), because of rapid conduction over
Bachmann's bundle, the circuit may be completed via the left atrial
myocardium and electrical connections between the coronary sinus
musculature and the left atrium. Thus, the mid-coronary sinus
electrograms may be earlier than the proximal coronary sinus. This
eccentric activation of the coronary sinus may give the impression
of an accessory pathway being present. It should be noted,
however, that mapping the fast pathway region (as explained above) will
always show earlier activation than the earliest site in the coronary
sinus. It should also be noted that eccentric activation when
occurring in AVNRT occurs only with typical AVNRT (retrograde fast
pathway with left atrial completion of the circuit).

Figure 4: In
children, at times there is anatomic continuity of
the crista terminalis and the eustachian ridge in the right
atrium. This anatomic fact paired with the relative rapid
conduction times between the atria creates unusual coronary sinus
activation patterns in some children with AV node reentry. The
propagated impulse after exiting from the fast pathway may travel to
the left atrium through Bachmann's bundle region and then enter the
coronary sinus in its mid or distal portion through muscular
connections and then travel back to the compact AV node region (slow
pathway). In these instances, accurate mapping of the fast
pathway will still show the earliest atrial activation, however, the
coronary sinus activation may be eccentric.

Wolff-Parkinson-White (WPW) and
Related Syndromes - Relevant Anatomy

Wolff-Parkinson-White syndrome results when an
accessory pathway is present that electrically connects ventricular and
atrial myocardium and gives rise to reentrant supraventricular
tachycardia. The electrocardiographic recognition and
electrophysiological approach to management of this syndrome in
children is discussed elsewhere.10
Specific causes for
difficulty with ablation in children with WPW are discussed in another
manuscript in this series. In this paper, I will discuss
the anatomy of the variants of this syndrome including unusually
located accessory pathways and the anatomical basis for pathways that
exhibit unusual electrophysiological behavior.

Specific
anatomic variants

Accessory pathways in children, as well as adults,
is sometimes classified into endocardial and epicardial types. In
a strict anatomic sense pathways are always epicardial. That is
the atrioventricular connection occurs exterior to the fibrous annulus
(tricuspid or mitral). However, most of these pathways can be
ablated either on the pathway itself or its atrial or ventricular
insertion with standard endocardial techniques. In a few
exceptional circumstances the pathway is truly epicardial in that the
atriovenous connection occurs through another epicardial structure
(usually the cardiac veins). In an analogous fashion, pathways
that course through the fibrous trigone represent situations where
standard endocardial ablation is unlikely to be successful for an
anatomic reason.

Venous pathways

Muscular
extensions (cardiac syncytial) into most
cardiac veins including the superior vena cava, pulmonary veins and the
coronary sinus exist.11,12
This myocardial extension into the
coronary sinus is electrically continuous at one or more points to the
right and left atria (Figure 5).
The coronary sinus muscular
sleeve usually extends about 4 cm into the vein up to the junction with
the great cardiac vein (insertion point of the vein of Marshall and the
posterolateral ventricular vein). In some patients this muscular
sleeve may extend into one of the ventricular venous branches (middle
cardiac vein or posterior veins). In a few individuals this
muscular extension into the ventricular veins interdigitates with
ventricular myocardium, thus forming a true atrioventricular bypass
tract connecting the atrium and ventricle electrically via the muscular
sleeves of the coronary sinus and ventricular veins. Since the
ventricular veins and coronary sinus are "epicardial" ablation requires
a modified approach. Based on understanding the anatomy of these
connections either ablation of the ventricular insertion within the
middle cardiac vein or circumferential isolation of the ventricular
vein (middle cardiac or posterior vein), isolation of the coronary
sinus or ablation of the atrial insertions of the coronary venous
musculature can be attempted. Each of these approaches has
benefits and difficulties that can be readily understood from the
underlying anatomy. Ablation at the ventricular insertion site
often is deep within the middle cardiac vein and injury to the
neighboring posterior descending artery or branch may occur. While
isolation of the coronary sinus or disarticulation by ablating
each atrial insertion has been done it is a technically very difficult
procedure as multiple atrial connections typically exist. When
this is accomplished pacing (or a spontaneous rhythm) arising in the
coronary vein musculature may still show pre-excitation; however,
reentrant tachycardias should no longer be possible. An approach
of ablating more proximal in the ventricular vein attempting to isolate
that branch alone (middle cardiac vein, diverticulum or posterior vein)
is often an anatomic approach taken in these situations.

Figure 5: Myocardial
extensions exist in various cardiac veins
and are consistently seen around the proximal coronary sinus.
Note, at the transition of the coronary sinus to the great cardiac vein
at the region of the posterior coronary vein, there is an abrupt
termination of the myocardial sleeve (see text for details). Courtesy
of Dr. Anton Becker, Netherlands.

Trigone
pathways

While
technically atrioventricular connections
(accessory pathways) should be possible at any point along the
atrioventricular annulus, an exception occurs in the region of the
aortic mitral continuity. Thus, left anteroseptal accessory
pathways (and left anterior) are exceedingly rare compared to other
locations. This is because of the unique position of the aortic
valve preventing direct atrioventricular connections as could occur
across the annulus in other locations. For accessory pathways to
occur here, the cardiac muscle (pathway) would have to apparently cross
the central fibrous trigone. While this may occur another
possibility more evident from recent case description is a connection
from the atrial myocardium to the supra-aortic valvar myocardial
extension (see below) under ventricular
tachycardia and from there to
the ventricular myocardium.13
The situation therefore is analogous to
epicardial pathways that involve the cardiac veins. Here again, a
non-annular structure with a myocardial sleeve forms part of the course
of such pathways. Ablation of such pathways can occur at the
atrial or ventricular insertion but is typically best accomplished in
the right or noncoronary cusp of the aortic valve. Care must be
taken to avoid damage to the AV node (avoid sites with large atrial
signal), the aortic valve itself or the right coronary artery.

Appendage
pathways

Another
non-annular anatomic situation where atrial
and ventricular myocardial tissue is apposed is with respect to the
appendages. Accessory pathways that involve electrically active
myocardial connection occur between the appendages and the underlying
ventricle. Although rare, when seen, these connections constitute
an accessory bypass tract as conduction can proceed through these
fibers independent of the AV node. When such tracts are diagnosed
in children, the optimal approach is to ablate in a circumferential
manner isolating a portion of the left or right atrial appendage close
to its ostium. While the actual connection may also be targeted
deep in the appendage, the risk of perforation in children is
high. Targeting the ventricular insertion is typically futile as
it is often multiple.

Anatomy of physiologically
distinct accessory pathways

Some accessory
pathways may be located in the
typical atrioventricular annular location, however, demonstrate unusual
physiological properties such as decremental conduction or lack of
typical responses to decremental atrial pacing etc. The nomenclature
for these pathways is confusing with historical and contemporary usage
significantly different.14
The anatomy of these pathways is briefly
reviewed below.

Mahaim fibers

In
present electrophysiology terminology, a Mahaim
fiber is a true accessory bypass tract connecting the free wall of the
right atrium to either ventricular myocardium or the infra-Hisian
conduction system. Unlike a typical accessory pathway, however,
Mahaim fibers have AV node-like characteristics near the annulus and
connect to relatively apical myocardium or the infra-Hisian right
bundle-branch system through an insulated fascicle similar to the His
bundle/right bundle-branch.
These
pathways, when they occur in children with
relatively rapid AV nodal conduction, can be exceedingly difficult to
recognize as accessory pathways.15,16 They are characterized by the
absence of retrograde conduction and early ventricular activation
during atrial pacing occurring away from the annulus often at an
identical site as the right bundle exit via AV nodal conduction. This
is because these pathways may directly insert into the right
bundle or the moderator band. The absence of retrograde
conduction and the absence of dual AV nodal physiology distinguished
these structures from true duplication of the AV node, a rare
entity. Because these structures typically do not insert into the
ventricular myocardium near the annulus mapping the earliest
ventricular activation is of minimal utility. A His bundle or
Purkinje like potential associated with Mahaim fiber conduction should
be targeted, preferably near the annulus for best results.
Understanding the anatomy of these tracts will aid the ablation
procedure in children complicated by their small heart size and ease
with which mechanical trauma (bumping) can lead to transient loss of
conduction and prevention of further mapping.17

Retrograde
Decremental Pathways

While Mahaim
fibers do not conduct retrograde,
pathways that conduct either exclusively retrograde or bidirectionally
occur and are responsible for the syndrome labeled PJRT (permanent form
of junctional reciprocating tachycardia). These pathways are
distinguished from Mahaim fibers by having the ventricular and atrial
insertions close to the annulus and conducting retrograde. There
is no relationship anatomically with these pathways and the normal
conduction system. For unclear reasons, retrograde decremental
pathways are often found in the region of the pyramidal space,
close to the coronary sinus ostium in the posterior right or left
atrium.18 Pathways
associated with Ebstein's anomaly presumably
because of the long course traversing the atrialized ventricular
myocardium give rise to the long conduction times and decremental
properties.

Nodoventricular
/ Nodofascicular Pathways

Unlike a Mahaim fiber that connects atrial
myocardium to the ventricular myocardium, nodoventricular pathways
connect compact AV nodal tissue either to the fascicular system (right
or left bundle branch) or the ventricular myocardium. For these
pathways to occur and participate in tachycardia, some form of
longitudinal dissociation in the AV node is required, that is antegrade
conduction through the AV node and then down the
nodoventricular/fascicular tract and then retrograde to AV nodal tissue
occurs over a conduction interval too short to be allowed by the usual
refractory period of the AV node. The anatomic basis for such
dissociation or in fact a convincing example of the anatomy/pathology
of these connections is lacking and has prompted some
electrophysiologists to question their existence.19,20 Although
the anatomic delineation of these pathways has not been clearly
established in the literature, physiologically, some patients have
pre-excitation, however, with necessary conduction through the AV node
to effect the pre-excitation. Unlike fasciculoventricular tracts
(see below), tachycardia can be induced with these arrhythmias and
reset from both the ventricle (without entering the compact AV node) or
the atrium (only by entering the compact AV node).21

Fasciculoventricular
Tracts

Normally, there is a fibrous sleeve of insulation on
the penetrating bundle of His and the right and left bundle branches
until this sleeve disappears and the bundle branch tissue connects to
the ventricular myocardium (bundle branch exit). In some
patients, there is a breach in this insulation and ventricular
excitation occurs closer to the base on the septum either directly from
the His bundle or from the proximal bundle branches. This is
termed a fasciculoventricular connection or pathway. The reader
should note that these do not represent true atrioventricular accessory
bypass tracts since anatomically there is no direct connection from the
atrium to ventricular myocardium. Importantly, when the AV node
blocks, there is no pre-excitation and progressive decrement in AV
nodal conduction is not associated with progressive pre-excitation as
seen with typical atrioventricular bypass tracts. The pediatric
electrophysiologist should be cognizant of this anatomic variant and
differentiate these electrophysiologically from true bypass tracts
since fasciculoventricular tracts are not associated with tachycardia
and should not be targeted for ablation.

Atrial Flutter and Atrial
Fibrillation

In the pediatric population, in patients without
prior cardiac surgery or congenital heart disease, atrial flutter and
atrial fibrillation are uncommon. While atrial flutter is one of
the most frequent causes of fetal tachycardia, after birth, the
arrhythmia is rarely seen without coexisting cardiac disease.
Similarly, atrial fibrillation in the pediatric age group is unusual
without coexisting disease such as the Holt-Oram syndrome or with
familial/inherited atrial fibrillation. From the anatomic
perspective, a few important differences are significant with pediatric
ablation and are outlined below.22

The
Cavo-Tricuspid Isthmus

The
cavo-tricuspid isthmus also referred to as the
subeustachian isthmus is the critical zone for the typical atrial
flutter circuit.23 This is
the region of atrial myocardium between the
tricuspid valve and the inferior vena cava including the region of the
Eustachian ridge.24 In
infants and young children, the subeustachian
isthmus often has a paucity of atrial myocardium and a distinct
subeustachian pouch forms. Younger children tend to also have a
relatively large thebesian valve.25
Subeustachian pouches
tend to be seen in patients (even in adults) who have a prominent
thebesian valve (Figure 6).
Thus, with ablation of the
subeustachian isthmus in children, care should be given to avoid
perforation or coagulum formation in this pouch.
Recognizing the occurrence of this pouch is also
important when attempting to cannulate the coronary sinus including for
cardiac resynchronization procedures, and this issue if explained more
fully in another manuscript in this series.

Figure 6: Anatomic
dissection showing the complex relationships
between the right and left ventricular outflow tracts (Superior
view). Note that
the ascending aorta is very close to the SVC and is to the right of the
patient's body when compared to the right ventricular outflow tract and
pulmonary artery. The atrial appendages (RAA and LAA) drape over
the outflow tracts with the left ventricular outflow tract and
ascending aorta being closer to the RAA and the right ventricular
outflow tract and pulmonary artery being in proximity to the LAA.

Pulmonary Vein
Anatomy

When
ablating atrial fibrillation in children, the
pulmonary veins are typically isolated. However, the veins tend to be
smaller in children and the sleeve of myocardium entering the vein is
also smaller.26 Further,
conduction velocities tend to be fast and the
typical decrement or delay in conduction seen at the pulmonary vein
ostium may not be as apparent.27,28 Great care must be taken to perform
all ablation in the atrium (proximal to the ostium). This is
because ablation at this site is required to isolate the heterogenous
arrhythmogenic tissue formed at the ostia and more importantly to avoid
pulmonary vein stenosis. When ablating within the pulmonary vein,
endothelial proliferation dominates and stenosis can occur, whereas
ablating at sites of more atrial myocardium in addition to causing
endothelial proliferation produces aneurysmal dilation of the
myocardial layer making stenosis unlikely.29,30

Pulmonary Vein
Branching

Because of the small pulmonary veins and branches,
acute occlusion of one of the venous branches may occur during
ablation. When venography is performed, it can be difficult to
know whether a branch has been occluded or did not exist at all. A
simple anatomic rule of pulmonary vein branching can assist the
operator in making this decision. Daughter veins that tend to
coalesce into a parent trunk have a consistent relationship in terms of
the diameter or circumference. The diameter (or circumference) of
the daughter veins (tributaries) when added up always is greater (110%)
than the diameter (or circumference) of the main vein (Figure 7).

Figure 7: Prevalence of
Right Atrial Pouch: Children
more commonly, as well as some adults, have a
pouch in the subeustachian region frequently associated with a
prominent thebesian valve. These pouches may make ablation of
typical atrial flutter or cannulation of the coronary sinus challenging
in children.25

Ventricular Tachycardia in
Children - Relevant Anatomy

Although
ventricular tachycardia ablation in
children has gone from a rarely performed procedure to more frequently
done, it still remains most likely required in children with congenital
heart disease or other structural abnormalities. When significant
structural abnormality is present (cardiomyopathy, tetralogy of
Fallot), a defibrillator has often been placed, and the ablation
procedure performed to minimize frequent shocks. The use of
defibrillators in children and principles of radiofrequency ablation in
pediatric scar-related VT are covered elsewhere in these discussions
and this supplement. In this paper, we will expand on the
anatomic basis of the two most common ventricular tachyarrhythmias
found in children with otherwise structurally normal hearts.

Outflow Tract
Ventricular Tachycardia

Outflow tract
ventricular tachycardia is
characterized by exercise-related wide QRS tachycardia that mostly
occurs in active older boys and less commonly as paroxysmal VT in
postmenarchal girls. In a few children with right ventricular
outflow tract tachycardia, underlying cardiomyopathy, particularly
arrhythmogenic right ventricular cardiomyopathy is found, but the vast
majority have no obvious structural heart disease. An accurate
understanding of the underlying anatomy of the outflow tracts is
critical to safe catheter manipulation, mapping, and ablation of this
arrhythmia in children. Pediatric ablationists should familiarize
themselves with the relative positions of the right and left
ventricular outflow tracts, myocardial sleeves that extend beyond the
semilunar valves, and knowledge of the coronary arterial systems
relative to both the left and right ventricular outflow tracts.

"Right" vs.
"Left" Outflow Tracts

The right
ventricular outflow tract courses anterior
and leftward in the body and at the level of the semilunar valves is to
the left of the left ventricular outflow tract. The angle formed
between these two outflow tracts varies with age from about 60° in
infants to about 90° in older children. Thus, when mapping an
outflow tract tachycardia in the right ventricular outflow tract,
novice ablationists may consider mapping the left ventricular outflow
tract if earlier sites of activation are found as the catheter is
positioned more leftward, however, left of the right ventricular
outflow tract is some left ventricular myocardium and the mitral
annular region anterolaterally. Earlier sites of activation,
particularly if the early signal is far-field in nature when located rightward and posteriorly should
prompt consideration of left
ventricular outflow tract mapping. Similarly, when mapping the
left ventricular outflow tract, leftward and anterior activation sites
should prompt reconsideration of meticulous mapping of the posterior
right ventricular outflow (a site where catheter positioning is not
straightforward) (Figure 8).

Figure 8:Relationship
of Pulmonary Vein Branches to the Parent Trunk: A
consistent pattern of pulmonary vein branching is
noted. The diameter (or circumference) of the parent trunk is
slightly smaller than the diameter (circumference) combined of the
branches (see text for details).

Myocardial
Sleeves Extending Beyond the Semilunar Valves

Myocardial
sleeves have been well described that
extend beyond the semilunar valves to various lengths endocardial to
the great arteries. While these occur in all age groups, the
relative lengths of these myocardial sleeves appear to be longer in
infants and young children.31
When mapping the right or left
ventricular outflow tracts, it is important to include supravalvar
mapping including the regions close to the ostia of the main coronary
arteries in the case of the aortic valve.32
The quality of these
signals is different in many cases from the ventricular myocardium
because of delay in conduction often seen at the level of the semilunar
valves. It should be noted that simply the presence of signals
above the semilunar valves is not necessarily arrhythmogenic (in
present thinking), and this myocardium may be a bystander that is
passively activated from a focus of tachycardia below the
valve.33

Coronary
Arterial Anatomy

Because the
pulmonic valve is cephalad and leftward
of the aortic valve, the posterior portion of the right ventricular
outflow tract just above and at the level of the pulmonic valve is very
close to the left main
coronary artery. While it is common for
ablationists to consider performing coronary angiography when ablating
in the left ventricular outflow tract, the proximity of the ostium and
initial course of the left main coronary artery is closer to the a
catheter position to deliver radiofrequency energy in the posterior
right ventricular outflow tract close to the pulmonic valve. Because of
the relatively shorter angle formed in very young patients
and the lack of thick myocardial separation, particular care when
ablating at these locations in children is required. When doubt
exists, either intracardiac echocardiography to carefully document the
separation between the ablating catheter and the coronary arteries (if
technically adequate views obtained) or coronary angiography should be
performed.34

Fascicular
Ventricular Tachycardia

Exercise-related ventricular tachycardia in
structurally normal hearts that exhibit a right bundle branch block
superior access morphology is usually mapped and ablated in the region
of the left posterior fascicle (Belhassen's VT). While the heart
is typically structurally normal, tissue that traverses the left
ventricular cavity from the septum to the free wall is sometimes found
close to the region of successful ablation. These have been
variously referred to as false tendons, interpapillary muscle chords,
left ventricular chords, or lancisi fibers. While such structures
are commonly found even in patients without ventricular tachycardia
that has been documented when clearly recognized with an imaging
modality (echocardiography), their presence can guide the ablationist,
particularly when tachycardia has been difficult to induce.

Summary

An
important trend in contemporary electrophysiology
is anatomy-based ablation. Although the pediatric
electrophysiologist still needs to clearly understand the physiological
principles underlying various mapping, diagnostic, and ablation
maneuvers, an appreciation of the underlying cardiac anatomy has become
critical.
Understanding cardiac anatomy will help minimize
complications including collateral damage to the AV conduction system,
pulmonary veins, and neighboring structures in the pyramidal space of
the heart. Further, an appreciation of the complex anatomy of the
ventricular outflow tracts, particularly in children allows accurate
correlation between mapping, electrocardiographic, and imaging
data. Such understanding again decreases the likelihood of damage
to the coronary arteries while facilitating successful ablation.

13. Gami AV, Venkatachalam KL, Friedman P, Asirvatham S.
Successful Ablation of Atrial Tachycardia in the Right Coronary Cusp of
the Aortic Valve in a Patient with Atrial Fibrillation: What is the
Substrate? Journal of Cardiovascular Electrophysiology. 2008;in press.

14. Wellens H. The preexcitation syndrome.
In: Electrical Simulation of the Heart in the Study and Treatment of
Tachycardias. Baltimore: University Park Press; 1971:97-109.